The present invention relates to a sensor for sensing the density of a color toner deposited on an image carrier with a light emitting element and a light receiving element, and a color copier or similar image forming apparatus using the same.
A copier, printer or similar image forming apparatus develops a latent image formed on the surface of a photoconductive element or image carrier by use of a developer stored in a developing unit and containing a toner. Because the toner is sequentially consumed due to repeated development, a fresh toner must be replenished into the developer in order to maintain the density of image constant. For this purpose, it has been customary to locate a reference chart having a preselected density in the vicinity of a glass platen to be loaded with a document. A reference pattern representative of the reference chart is formed on the image carrier by exposure and development. The density of the reference pattern is optically sensed in order to control the replenishment of the toner. This control scheme stems from the fact that the toner concentration of the developer varies in proportion to the developing density, i.e., the amount of toner deposited on the image carrier. Specifically, the sensed density of the reference pattern is compared with the preselected density. If the sensed density is higher than the preselected density, the replenishment is interrupted or reduced in amount. If the former is lower than the latter, the replenishment is resumed or increased in amount.
A red, blue or similar monocolor copier is available today. This kind of copier is operable with developing units respectively storing a black toner and a color toner and replaceable with each other, or with such developing units fixedly arranged side by side and selectively used, or with a full-color developing unit.
As for the black toner, it is a common practice to sense the density of the reference pattern by use of optical sensing means made up of a light emitting element and a light receiving element. A plane containing the optical axes of the light emitting and light receiving elements is coincident with a plane containing a normal extending from the image carrier. Hence, the light receiving element senses a regular reflection from the light emitting element. However, the color toner diffuses light incident thereto. Hence, the image carrier and color toner differ little in reflectance from each other. This makes it impossible to set up a correlation between the density of the color toner and the output voltage of the light receiving element, i.e., the quantity of diffused reflection. Consequently, it is difficult to sense the density of the reference pattern formed by the color toner.
In light of the above, Japanese Patent Laid-Open Publication No. 61-209470 discloses a toner density sensor in which at least one of the light emitting element and light receiving elements is rotatable in the plane containing their optical axes. In this sensor, the light receiving element receives the regulate reflection in the event of development using the black toner, or receives the diffused reflection in the event of development using the color toner. Japanese Patent Laid-Open Publication No. 62-164066 teaches a replenishment control method using an infrared photosensor whose output characteristic resembles a curve of secondary degree. As for the monocolor toner, the method effects control in a color characteristic range in which the output of the photosensor increases with an increase in image density, and limits the replenishment when the sensor output rises above a preselected value. Further, Japanese Patent Laid-Open Publication No. 62-209476 proposes a method using two light receiving elements which are respectively assigned to the regular reflection and diffused reflection, so that the replenishment can be controlled on the basis of a difference between their outputs.
However, the prior art color density sensing methods and devices stated above have some problems left unsolved, as follows. The plane containing the axis of the image carrier and the axes of the light emitting and light receiving elements is coincident with the plane containing the normal of the image carrier. The angles of the light emitting and light receiving elements are varied within the above plane. In this condition, the reflection from a color toner image formed on the image carrier is a diffused reflection, and is therefore extremely small in quantity. To sufficiently sense such a reflection, it is necessary that the two elements be positioned close to the image carrier (surface to be sensed), or that their light emitting surface and light receiving surface by increased in size. This kind of approach, however, causes much of the regular reflection from the toner image to be incident to the light receiving element together with the diffused reflection, preventing the toner density from being accurately sensed. In addition, the above approach makes it necessary to assemble the mechanism in a limited space, and complicates the construction of the apparatus.